The present invention relates to direct bonding of a metal to a metal or ceramic substrate. More specifically, the invention relates to a method of fabricating large area, blister-free assemblies of metal direct bonded to a variety of metallic and ceramic substrates.
U.S. Pat. Nos. 3,766,634 issued Oct. 23, 1973 to G. L. Babcock et al and 3,994,430 issued Nov. 30, 1976 to D. A. Cusano et al (both assigned to the same assignee as the present invention) describe several methods of direct bonding metallic and non-metallic refractory materials to metals which are useful in the fabrication of integrated circuit modules, and in providing current carrying electrical conductors on insulating substrates with high thermal conductivity paths to a heat sink. Examples of specific applications of direct-bonded metal-ceramic assemblies are provided in the aforementioned patent to D. A. Cusano et al. The application of assemblies made up of direct-bonded metal to ceramic in hybrid, high power electronic circuits is described by Y. S. Sun and J. C. Driscoll in "A New Hybrid Power Technique Utilizing a Direct Copper to Ceramic Bond", IEEE Transactions on Electron Devices, V. ED-23, No. 8, August 1976.
In the past, "thick-film" and moly-manganese processes have been among the methods employed to bond metal to ceramic. These methods typically require formation of an intermediate bonding layer at the metal-ceramic boundary. The metal-ceramic assemblies made by these methods are, however, unsatisfactory for high power and/or high frequency applications due, in part, to the low electrical and thermal conductivities of the intermediate layer. The assemblies are unsuitable for high frequency use because of such frequencies current flows in the high resistance intermediate layer at the metal ceramic interface. Additionally, the high thermal impedance of the intermediate layer diminishes the ability to dissipate heat in the substrate. Moreover, the conductive metal layer fabricated in accordance with these methods is typically less than 0.001 inch thick, thus severely limiting the usefulness of the assembly in applications wherein high current surges occur. Typically, the thin conductive layer must be built up by electrodeposition of additional metal.
Direct-bonded metal to ceramic is particularly useful in high power and high frequency applications. The eutectic metal-to-ceramic bond (to be more fully described hereinafter) is characterized by the absence of a readily identifiable intermediate layer at the metal-ceramic interface. The absence of the intermediate layer provides good electrical and thermal conductivity at the metal-ceramic junction. Good electrical conductivity in this region permits satisfactory high frequency operation, while good thermal conductivity allows the use of the ceramic for direct heat-sinking. Beryllia (BeO), which is the only material other than diamond that combines high thermal conductivity with superior electrical insulating qualities, is a particularly useful ceramic for applications requiring high heat dissipation. Direct bonding methods also permit bonding of metals of substantially greater thickness. For instance, copper material having a thickness as great as 0.25 inch has been successfully direct-bonded to beryllia.
A drawback associated with direct bonded assemblies fabricated by direct bonding methods is that large area metal-substrate bonds cannot be reproducibly formed without the occurrence of blisters in the metal-substrate interface. The cause of the blistering phenomenon is not completely understood, but is believed to be due to entrapment of gases, or evolution of gases from contaminants during the direct bonding operation. Generally, the substrate area to which metal can be reproducibly direct-bonded without blistering is limited to less than one square inch. The blistered areas present an uneven metal surface which is not sufficiently flat for reliably soldering electronic components thereon and thus must be sanded or filed to the required flatness. Moreover, since there is a bonding discontinuity at the metal-substrate interface, where blisters appear, thermal dissipation through the assembly is decreased. The reliability of electrical contacts is also decreased, since during thermal cycling solder joints which connect electronic components to the copper foil may fracture due to uneven expansion or contraction of the copper in the region of bond discontinuity compared to unblistered assembly portions.
The present invention provides a method for reproducibly direct bonding large area metallic members to a variety of substrates without blisters. The metal bonds intimately to the substrate, providing a flat surface for soldering electronic components. The intimate metal-to-substrate bond permits superior thermal dissipation and electrical conduction. Blister-free, direct bonded, metal-to-ceramic assemblies as large as 2.times.2 inches have been produced by the method of the present invention. Larger areas assemblies may be produced if desired.